This invention relates to core constructions of nuclear reactors, and more particularly it is concerned with a core construction of a nuclear reactor in which four fuel assemblies arranged in a square adjacent a control rod are counted as a core unit and a multiplicity of such core units are arranged to provide the core construction.
It is necessary that the fuel assemblies arranged to provide a core of a nuclear reactor be replaced periodically by new fuel assemblies depending on the operating conditions of the nuclear reactor. The core of a nuclear reactor is usually designed such that 1/4 of the total fuel assemblies are replaced by new ones when replacement is carried out periodically. Such core construction is referred to as a four batch replacing core. This type of core construction suffers the disadvantage that since each batch of four fuel assemblies has its fuel assemblies replaced by 1/4, it is necessary to shut down the reactor three times before one fuel assembly is burned up.